A vector network analyzer (VNA) is a reflectometer-based electronic instrument that can be used to measure the frequency response (magnitude and phase) of a device under test (DUT) such as an electrical network, component, circuit, or sub-assembly. A VNA makes use of a frequency sweeping source or stimulus, directional couplers, and one or more receivers that provide ratioed amplitude and phase information such as reflection and transmission coefficients. Directional couplers are widely used in VNAs to separate the waves that are incident on, reflected from, and transmitted through a DUT. A directional coupler diverts signal power from a main line to which it is coupled. The wave-separation property of directional couplers makes them fundamental in VNA design. It can be desirable for a directional coupler to have good wave-separation across wide frequency ranges. This desire is driven at least in part by the fact that wide-band directional couplers make possible the design of wide-band VNAs. However, couplers in accordance with the prior art have limited bandwidth.
Referring to FIG. 1, a VNA 100 is shown connected to a DUT 110. The VNA makes use of a reflectometer receiver 102 for signal separation and detection. A reflectometer receiver 102 uses a local oscillator (LO) 108 to mix a radio frequency (RF) signal or stimulus from an RF source (also referred to herein as an input) 106 down to a lower intermediate frequency (IF) signal. The LO 108 is either phase-locked to the RF or the IF signal so that the reflectometer receiver 102 is tuned to the RF signal present at the RF source 106. The incident wave and related signal source paths are indicated by an “a” and the reflected wave and related signal source paths are indicated by a “b”. The IF signals are bandpass-filtered to improve sensitivity and dynamic range and are provided to a processor/display 112.